The El Cajon Superior Court jury was unable to reach a verdict, and the judge declared a mistrial Monday.
Jurors said they were leaning 11-1 in favor of convicting 23-year-old Anthony Miller.
Miller is accused of participating in a liquor store robbery in which co-owner Heather Mattia and store employee Firas Eiso were shot in the head execution-style.
Co-defendant Jean Pierre Rices pleaded guilty to murder and has admitted shooting the pair. A separate jury is considering whether to recommend the death penalty.
Miller's defense attorney says a new trial date is set for September.